Combine the milk, chocolate, cocoa powder, agave and a pinch of salt in a medium saucepan and set over medium-low heat. Cook, whisking constantly, until the chocolate melts and the cocoa dissolves, 6 to 8 minutes. Pour the chocolate mixture into a medium bowl. Stir in the chia seeds and vanilla. Let cool to room temperature, about 1 hour.
Cover and refrigerate the mixture until chilled and the chia seeds are plumped, about 1 hour more, stirring halfway through.
Divide the mixture evenly among four 3- to 4-ounce ice pop molds. Freeze until solid, at least 8 hours and up to overnight. Let sit at room temperature for a few minutes before unmolding.
The pops will keep in the freezer for up to 2 weeks.
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